Additive for stainless steel pickling baths



(t United States Patent 0 3,349,037 ADDITIVE FOR STAINLESS STEEL PICKLING BATI-IS Joseph C. Peterson, 3801 N. Meridian St., Apt. 402, Indianapolis, Ind. 46208 No Drawing. Filed Dec. 14, 1966, Ser. No. 601,568 6 Claims. (Cl. 252146) ABSTRACT OF THE DISCLOSURE This invention provides an additive for stainless steel pickling baths which conserve acid and reduces the corrosion and health hazards caused by the escape of noxious fumes from the pickling bath. The additive consists essentially of from 20-30 parts by weight of sodium-2-ethylhexyl sulfate, from 25-30 parts by weight of propylene glycol and from 0.5-1 part by weight of lignin sulfonate.

CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of my copending application Ser. No. 395,934 filed Sept. 11, 1964, now abandoned.

BACKGROUND This invention relates to a novel process for pickling stainless steel and to novel compositions useful in that process.

Pickling is a term used in the metal industry to define a process whereby metal oxides and scale are removed from the metal surface prior to a particular fabrication step. In general, stainless steels are difiicult to pickle and it has been the custom of the industry to use a two-step pickling process for these steels. The first step usually involves immersing the steel in an 8-12% by volume 66 degree B. sulfuric acid bath at temperatures in the range 160-180 degrees F. for periods of time ranging from to 45 minutes. The second step involves immersing the stainless steel from which gross scale has been loosened, in a cleaning and whitening solution containingfrom 8 to (by volume) 40 degree B. nitric acid and 1.5- 3% (by volume) 70% hydrofluoric acid at temperatures in the range 120-160 degrees F. for periods of from 2-30 4 minutes.

The second step of this process has long been a source of trouble to the steel industry. The combination of nitric and hydrofluoric acids is one of the most corrosive combinations known to man. The acid fumes from these pickling baths destroy steep supports and galvanized roofing. Furthermore, at temperatures of 140 degrees F. and above (some picking operations use hydrofluoricnitric baths at 200 degrees F.), the fuming of nitric acid itself becomes a health hazard. These fumes, which are nitric oxides, are extremely toxic. Workmen in the steel industry are, of course, aware of the hazards to lungs, skin and clothing associated with nitric-hydrofluoric pickling baths, and employment of such workers are standard wage rates is becoming increasingly ditficult.

In addition to the health and corrosion hazards associated with these stainless steel pickling baths, there is also the monetary loss suffered by the fuming of the acid baths.

The prior art processes do not provide a satisfactory solution to the above problems. In fact, the prior art does not seem to have recognized the problem, but rather has been directed to increasing the efficiency of etching baths for a single metal as in Australian Patent 210,640, or the brightening of a single metal such as aluminum as in U.S. Patent No. 3,119,726.

Therefore, it is an object of this invention to provide a stainless steel pickling additive which conserves acid and reduces the corrosion and health hazards caused by the escape of noxious fumes from the pickling bath.

It is a further object of this invention to provide a steel surface after pickling that is whiter and cleaner than that 5 provided by standard nitric-hydrofluoric acid pickling. Other objects of this invention will become apparent from the description which follows.

DESCRIPTION OF THE INVENTION The above objects are fulfilled by the provision of an additive for nitric-hydrofluoric or sulfuric pickling baths comprising sodium-2-ethylhexyl sulfate, propylene glycol and lignin sulfonate. The following table gives the permissible ranges of ingredients in my novel composition.

15 TABLE 1 Permissible range, Ingredient: parts by wt. Sodium Z-ethylhexyl sulfate -30 Propylene glycol -30 20 Lignin sulfonate 0.5-1

I customarily add from 39-55.5 parts of water to the mixture of the above ingredients. The sodium Z-ethylhexyl sulfate can, of course, be provided in other equivalent 25 forms such as the free acid or as another soluble cationic salt since the material is necessarily present in the acid bath as the free acid. I prefer to use propylene glycol in my novel pickling additive in order to prevent the ingredicuts from precipitating when stored in the cold, and to decrease foaming in the pickling bath, but it will be apparent to those skilled in the art that other solvents, which do not break down in a nitric-hydrofluoric acid bath and which lower the surface tension of the bath, can be used in place of propylene glycol in the above composition without departing from the scope and purposes of this invention.

In actual use, by novel pickling additive is usually added to the acid bath in an amount equivalent to 2% of the weight of the acid content of the bath. For example, 0 a stainless steel pickling bath is prepared containing 10% by volume of 70% nitric acid and 2% by volume of 70% hydrofluoric acid; total weight of the contents is about 2,700 lbs., 54 lbs. of a composition containing 1% lignin sulfonate, 30% sodium Z-ethylhexyl sulfate, 30% 5 proylene glycol and 39% water would be added to the pickling bath. Ordinarily, 2% of my novel additive composition will be sufiicient to provide the advantages to be delineated hereinafter but more may be used (up to 4%) for high temperature pickling. The pickling baths containing my novel additive can be used for either continuous or batch pickling of stainless steel. Among the stainless steels which have been successfully pickled by the above process are included N- 155, 303-MA (chrome-nickel), 300 (same steel with 55 molybdenum), 188 (same steel with titanium), 302-1, 304, 316, 5108, 301-1, 347, 304L, 3098 and 310S and all 400 series. In general, the novel pickling process of this invention manifests its superiority better with the tougher grades of stainless steel that are notoriously harder to clean. For instance, high nickel stainless steels which cannot normally be cleaned in nitric-hydrofluoric acid baths, are readily cleaned when the baths contain my additive. The above composition is, suprisingly, quite stable to attack by the nitric-hydrofluoric acid combination, a necessity since any organic breakdown products would tend to stain the steel and to tar up the bath, the rollers used to fabricate steel after pickling, etc.

The addition of my additive makes it possible to operate more efiiciently at a lower temperature and a lower total acid concentration and thereby decreases the cost of the operation. For example, where pickling of 302 DA stainless previously required 25-30% total acid, with my novel additive present, only 12% total acid was needed to achieve a comparable ickling quality. In another illustration of the advantages of the pickling additive of this invention, in using standard 15% total acid pickling baths, additions of acid were required every two hours to maintain the 15 total acid concentration. Addition of 2% of a composition coming within the scope of this invention required no added acid in an 8-hour period to maintain total acidity. The acid savings per ton of steel pickled when employing my novel pickling additive are in the range of 30-40% or even 50% per ton of steel pickled. In actual practice, the acid bath life has frequently been doubled, and twice the tonnage of stainless steel can be pickled in the bath in the same period of time.

In addition to the above advantages, there are others which are not so obvious. For example, the rubber rolls at the end of the pickling line no longer become slippery with scum. The quality of the pickled steel is also improved. There is no over pickling including embrittlement, pitting, etching, etc., such as is frequently present with conventional pickling baths when the hydrofluoric acid concentration increases proportionally because of the loss of nitric acid.

It is obviously an added advantage of my novel pickling additive that such additives can 'be present without harming the ap ernnce of the steel after pickling. Visual inspection of steel samples pickled in baths with and without my novel additive shows that steels picked in baths with the additive are of comparable brightness or brighter than steels pickled in conventional baths (without the additive).

This specification is further illustrated by the following specific examples.

Example I.Cntinuou.r pickling of stainless steel was passed through the bath with an in-time of about 45 sec. During the first 24 hours of operation, 168 gals. of 70% nitric acid, and 18 gals, of 70% hydrofluoric were added to keep the level of the bath constant. 4.8 gals.

of the additive specified above were also added to maintain its concentration. (In normal operation without my novel additive, 328 gals of 70% nitric acid and 48 gals. of 70% hydrofluoric acid are required to replace acid lost by foaming or vaporization or by mechanical loss.)

Over 3,000,000 lbs. of one type of steel were processed in a bath prepared as outlined above before it was necessary to dump the bath.

Example II.BaIch pickling A solution having a total volume of 3,480 ml. was prepared using 3,000 ml. of water, 370 ml. of 70% nitric acid, and 110 ml. of 70% hydrofluoric acid After thorough mixing an analysis of the solution for nitric and hydrofluoric acid showed:

Percent by weight Nitric acid 10.67 Hydrofiuoric acid 2.01

The remaining 3,450 ml. was divided into two lots of 1,725 ml. each. One lot was kept as a control, and to the other lot was added 6.53 g. of an additive having the following composition: 0.5 by weight of lignin sulfonate,

4 30% by weight of sodium 2-ethyl hexyl sulfate, 25% by weight of propylene glycol, and 44.5 water. Both solutions were heated to 142. F. and maintained at this temperature during the pickling process. Sample strips of eight different steels were pickled in the heated bath for a period of five minutes, washed in running water, scrubbed with a nylon brush, and dried on paper towels.

A similar test was carried out with five-minute pickling of five different steels in the same two baths but at a F. pickling temperature.

In twelve out of sixteen tests, the gage loss was greater in the bath without additive than with the bath containing our novel additive. In the other tests, the loss in the bath with additive was greater than that in the bath without additive by no more than .0005". While the acid action was increased, the weight loss was not increased. That is, the bath with the additive increases the efiicieney of the pickling process without attacking the metal itself.

In the above experiment, titration of the acids in the baths after pickling shows that the solution contains the following:

Bath with additive- Percent by weight Hydrofluoric 2.00

Nitric 10.61 Bath without additive- Hydrofiuoric 1.98

Nitric 10.14

Thus, there was a loss of only .01% hydrofluoric and .06% nitric acid in the bath containing the additive, but a loss of .03% hydrofluoric and .53% nitric acid in the bath without the additive.

Visual inspection of the steel samples pickled at 142 F. and 160 F. in the baths with and without my novel additive showed steels 301-1 and 302-1 pickled in the bath with additive were brighter than those pickled in the regular bath, and that with all the other steel samples except the 309s and 3105 samples at 142 F. and the 310s sample at 160 F., the brightness of the samples from the two baths were strictly comparable.

It can well be understood that the best way to determine the true value and success of a pickling process is to test the process against the standard processes in actual mill conditions. Thus, mill test evaluations were run in three representative steel companies and the results given in Tables II, III and IV showing the diiference in acid consumption, production and costs for an I-IF-HNO pickling bath with and without the additive for a one (1) month period and in Table V for a one (1) year period.

TABLE II.MILL #1 lbs. of stainless steel was pickled in a one month period in baths with the additive than in baths without the additive utilizing approximately 18.2% less of l-INO 6.92% less of HF, and 7.35% less of H 50 which amounted to a savings of 24% per ton.

The production was increased approximately 28% with my process using approximately 9% less HNO and 54.6% less HF which amounted to a savings of 41% per ton.

TABLE IV.MILL #3 (Figures for 1 month with and without the additive) Applicant's Standard Statistics process (with M ill process additive) (without additive) Total tons stainless steel processed 680 650 Consumption of H NO: (gal.)...- 960 2, 400 Consumption of HF (g 450 9 Consumption of additive (gaL) 40 Total cost $1, 070. 00 $2, 150. 00 Cost per ton $1. $3.

Over a one month period, production was increased approximately 5% with my process utilizing 60% less HNO 50% less HF and resutling in a savings of approximately 47%.

TABLE V.MILL #3 (Figures for a 1 year period with and without the additive) Applicant's Standard Statistics process (with Mill process additiv (without 40 additive) Total tons of stainless steel processed 67, 500 68, 000 Consumption of HNO; gal.) 105,000 310, 000 Consumption of HF 90, 000 142, 000

Over a one (1) year period production was kept apprixately the same to measure savings. With my process, 66% less HNO and 36.7% less HF was utilized which resulted in a savings of approximately 47%.

In all of the above mill production tests, each mill followed its normal operating procedures, adding about 2% by weight of the additive to the baths. In each mill, acid consumption and hence costs were greatly reduced. It was further found that the use of the additive greatly retarded the sludge build up in the tubs, and increased the life of the pickle tubs and equipment as much as 50%.

Thus, the present invention has met a long-standing need in the art for improving the conditions in pickling mills. The process is far more economical than any processes previously utilized in the industry. It is safer both for the equipment and the workman and has afforded a considerable advance in the art.

Our novel pickling additive, in addition to its use in continuous and batch pickling operation, can also be employed in electrolytic picking processes. This type of pickling is particularly efiective in treating chrome and chrome-nickel stainless steels. The usual decrease in acid loss and the usual increase in bath life found when my novel additive is employed in batch or continuous pickling are also found when it is used in electrolytic pickling.

While the invention has been disclosed and described in some detail in the foregoing description, the descriptions are to be considered as illustrative and not restrictive in character, as other modifications may readily suggest themselves to persons skilled in this art and within the broad scope of the invention, reference being had to the appended claims.

The invention claimed is:

1. An additive for acid pickling baths consisting essentially of 0.5 to 1 part by weight of lignin sulfonate, 20 to 30 parts by weight of sodium 2-ethylhexyl sulfate, and 25-30 parts by weight of propylene glycol, the balance being water.

2. A nitric-hydrofluoric acid pickling bath containing an additive in accord with claim 1, said additive consisting essentially of one part by weight of lignin sulfonate, 30 parts by weight of the sodium salt of 2-ethylhexyl-sulfate and 30 parts by weight of propylene glycol, the balance being water.

3. A nitric-hydrofluoric acid pickling bath containing the additive in accord with claim 1 wherein said additive consists essentially of 0.5% by weight of lignin sulfonate, 30% by weight of sodium 2-ethylhexyl sulfate, 25% by weight of propylene glycol and 44.5% by weight of water.

4. A nitric acid-hydrofluoric acid pickling bath containing the additive of claim 1 wherein from 2 to 4% by weight of said additive is added to a nitric-hydrofluoric acid pickling bath for stainless steel, said additive reducing the amount of acid required for pickling stainless steel.

5. The process which comprises: pickling stainless steel in a nitric-hydrofluoric acid bath to which has been added up to 4% by weight of a composition consisting essentially of from 0.5 to 1 part by Weight of lignin sulfonate, from 20 to 30 parts by weight of sodium 2-ethylhexyl sulfate, and from 25 to 30 parts by weight of propylene glycol, the balance of said additive being water; and rinsing and drying said stainless steel.

6. The process of claim 5 which comprises pickling stainless steel in a nitrichydrofluoric acid bath to which has been added 2% of the weight of the acid present of said composition; rinsing and drying said stainless steel.

References Cited UNITED STATES PATENTS 3,119,726 1/1964 King et al 156-19 FOREIGN PATENTS 118,071 6/ 1958 Russia.

LEON D. ROSDOL, Primary Examiner. M. WEINBLATI, Assistant Examiner. 

1. AN ADDITIVE FOR ACID PICKLING BATHS CONSISTING ESSENTIALLY OF 0.5 TO 1 PART BY WEIGHT OF LIGNIN SULFONATE, 20 TO 30 PARTS BY WEIGHT OF SODIUM 2-ETYLHEXYL SULFATE, AND 25-30 PARTS BY WEIGHT OF PROPYLENE GLYCOL, THE BALANCE BEING WATER. 